Production technology of p-xylene production by toluene methylation with selective carbon dioxide hydrogenation
[Objective]p-Xylene(PX)is an important aromatic product with the highest consumption among xylene isomers.It is widely used as a raw material for upstream production of several important chemical products.Carbon dioxide(CO2)is a major gas responsible for the greenhouse effect.Furthermore,in addition to methanol,CO2 and syngas also show great technical potential as methylation agent.The direct synthesis of PX by CO2 hydrogenation coupled with toluene methylation over a bifunctional catalyst has the advantages of atomic economy,green hydrogen storage,and CO2 utilization,but a complete techno-economic evaluation of process design and optimization strategy has not been performed.[Methods]Based on the selective catalytic results of CO2 hydrogenation coupled with toluene methylation,three toluene methylation catalysts with high conversion,high xylene selectivity,and high PX selectivity and their experimental results were chosen,and the process flow of PX production by CO2 hydrogenation coupled with toluene methylation was designed and simulated by software.The following was the process flow:the raw materials of the reaction were pretreated and compressed into the reactor,the reaction products were flashed four times to separate the gas from the liquid,some raw materials were circulated,and the liquid products were sequentially separated or purified to yield benzene,PX,o-xylene,and heavy aromatic hydrocarbons.Moreover,because of the various catalysts,RStoic,a stoichiometric reactor module,was employed to model the methylation reaction unit.Based on the characteristics of different catalysts,the conversion rate of each reactant was specified.Furthermore,based on controlling the same PX output,the raw material feed ratio of the reactor was also specified,and the xylene isomer was separated by reactive distillation.[Results]Analysis of the raw material cost,equipment cost,and energy consumption of the process flow corresponding to the three catalysts was conducted,and the results showed that the raw material of the unit PX product of the high-PX-selectivity catalyst process was only 72.6％and 58.9％of those of the other two catalyst processes and theCO2 consumption of the unit PX product was 27.3％and 44.7％of that of the other two catalyst processes.However,the high-xylene-selectivity catalyst process could yield more PX production through isomerization technology,and its energy consumption was also the lowest.Because reactive distillation was employed as the post-separation of xylene isomerization products,the high-xylene-selectivity catalyst process had a high raw material consumption rate and the highest energy consumption,but by isomerization technology,it had the highest PX production potential and showed the best economy through economic accounting of different PX production processes.[Conclusions]CO2 hydrogenation coupled with toluene methylation technology can enhance the conversion rate of raw materials,reduce the energy consumption of material circulation,and enhance the xylene and PX selectivities,which will greatly improve its technical economy.Furthermore,the production process with high PX selectivity is a green chemical process with broad development prospects for reducing carbon emissions in the environment,achieving carbon cycle,energy conservation,and emission reduction.
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